This invention relates generally to charging circuits used for charging lead-acid storage batteries, and more particularly to a circuit arrangement of a quick charger for lead-acid storage batteries of sealed type.
Lead-acid storage batteries of sealed type are advantageous for powered tools having no power cable because energy density is high when discharging characteristic is considered and because of inexpensive cost when compared with other secondary batteries such as nickel-cadmium batteries.
As is well known in the art, it is somewhat difficult to handle lead-acid batteries of sealed type, however, compared with lead-acid batteries of open type such as those used for automobiles, because sealed type batteries should not be overcharged for electrolyte thereof is apt to be insufficient due to overcharging. Furthermore, insufficient charging should also be avoided to derive desired power from such a sealed type battery. Such undesirable ways of charging, i.e. overcharging and insufficient charging, shorten the life of lead-acid batteries of sealed type. In order to meet such requirement in charging therefore, lead-acid batteries of sealed type have been subjected to so called long-time charging in which a battery is charged for 15 hours or so, so as to charge the same in an ideal manner without suffering from overcharge or insufficient charge. It is also known in the art that an ideal charging state corresponds to 105% of a previously discharged amount.
However, such a long-time charging is not practical for powered tools because powered tools, such as electric drills, are desired to be used again with a relatively short period of time for battery charging. A quick charging system for powered tools was developed recently, and known in the art. This prior art is disclosed in National Technical Report Vol. 27. No. 6, published December 1981 by Matsushita Electric Industry Co. Ltd., under a title of Small Sealed type Secondary Battery Quick Charge System written by Yoneji Koyama et al. According to this known system, a programmed time limit constant-current charging mode follows a normal constant-current charging mode, and after the programmed time limit constant-current charging mode, a trickle charging mode takes place. In the programmed time limit constant-current charging mode, the charging current is stepwise reduced successive so as to forcibly charge a battery by injecting a predetermined amount of charge irrespective of the amount of charge achieved by the former normal constant-charging mode. Although this quick charge system provides a less charging period, since the charging amount at the instant of transition from the constant-current charging mode to the programmed time limit constant-current charging mode is not necessarily constant, this system is apt to suffer from overcharge and/or insufficient charge.